1. Field of the Invention
The present invention relates generally to uninterruptible power supply (UPS) apparatuses and isolating method thereof. More particularly, the present invention relates to an UPS apparatus employing a high frequency (HF) switching power supply circuit to allow an isolation circuit to be made more compact and lighter and an isolating method thereof.
2. Description of the Prior Art
A basic function of an UPS apparatus is to keep supplying power to a load by using a commercial power source as energy source when the commercial power source is normal, and using a storage battery or the like as energy source when the commercial power source or the like is interrupted.
The USP apparatus ordinarily employs a transformer for isolation in its output region or input region. However, since as a transformer, a transformer for low frequency is used, in principle it inevitably takes volume and weight to some extent, which is a serious obstacle to making an UPS apparatus more compact and lighter.
Recently in many cases, in place of such a low frequency transformer, a HF switching power supply circuit for outputting HF alternating current (AC) power is employed wherein the isolation is made at the output side of the HF switching power supply circuit. More specifically, an isolation circuit comprising a HF transformer is provided on an output stage side of a HF switching power supply circuit. An isolating method by using this HF transformer makes a low frequency transformer unnecessary, allowing a HF transformer to be made more compact and lighter than by an isolating method by using a low frequency transformer.
However, transformer loss is increased when the transformer is excited by the HF current. This transformer loss includes iron loss due to magnetic materials, copper loss due to windings or the like. When using a HF transformer, such losses not only reduce power conversion efficiency but also present such problem as countermeasure to heat radiation for the losses or the like.
In addition, problems are pointed out concerning electromagnetic interference problems, for example, that display image of a computer is affected by leakage flux of the HF transformer and that other electronic devices receive obstacle noise due to radiation electromagnetic field from the HF transformer.
On the other hand, in theory, even a HF transformer requires such heavy members as windings made of copper wire coils and cores made of magnetic materials, such as ferrite or cobalt amorphous, which are obstacles in making an UPS apparatus more compact and lighter.